Electromagnetic control means



Oct. 31, 1967 Filed Dec. 31, 1964 9 L4 z] La.4 IE KL E o2 Ll 87 Z Emma?INVENTOR. OWEN 5. STEELE Jim nus f 514)"? Afforzgvs United States PatentO 3,350,609 ELECTROMAGNETIC CONTROL MEANS Owen S. Steele, Toronto,Ontario, Canada, assignor, by mesne assignments, to A. O. SmithCorporation, Milwaukee, Wis., a corporation of New York Filed Dec. 31,1964, Ser. No. 422,739 13 Claims. (Cl. 317-123) This invention relatesto electromagnetic control means and particularly to an energizing andde-energizing control for an electromagnetic means such as a liftingmagnet, power solenoid or other similar device having a substantiallyhigh inherent self-inductance.

Lifting magnets of the electromagnetic variety and similar devices of ahighly inductive characteristic will produce a high voltage upondisconnection of the coil or winding from the source of energizingcurrent. The high voltages are capable of causing severe arcing andrelated problems in the circuit of the magnet as well as the life of thewinding insulation. Circuits have therefore been developed to compensateor reduce the effects of the voltages. Such circuits preferably alsoprovide for a rapid discharge of the stored energy in the coil to permita rapid drop of the material from the lift mechanism and also permitopposite momentary energization of the lifting magnet such that it issubstantially devoid of magnetic flux at reset.

' The present invention provides a control for a lifting magnet or thelike in a relatively simple and highly reliable controller.

Generally, in accordance with the present invention, a discharge circuitis permanently connected across the magnet and is operative immediatelyupon opening of the lift contacts. Generally, the discharge circuitincludes a discharge bridge having nonlinear resistors in selectedbranches. The output of the bridge circuit operates a dropping relayhaving contacts operable to rapidly reduce the current to zero and thenreverse the current and pass a demagnetizing current through the device.After a selected demagnetizing current period, the relay automaticallyopens to return the system to standby.

During the initial discharging period, the nonlinear resistorautomatically adjusts the resistance so that the voltage across themagnet is kept to a reasonably low peak value. As the voltage decreases,the resistance value of the nonlinear resistor increases with a relatedincrease in the voltage differential applied to the dropping relay toprovide the previously described sequence function. The nonlinear andother resistors of the bridge circuit in the present invention arepreferably selected such that only very minimal current passes throughany portion of the bridge circuit during normal lifting excitation.Further, the circuit is polarized to protect the nonlinear resistors ofthe discharge circuit. Thus, if the input power is connected withreverse polarity, the circuit will not be energized.

The present invention thus provides for a very rapid controlleddischarge of the stored energy with an automatic two-step sequencing,which maintains the voltage at the lowest minimum value consistent witha high speed operation. The discharge circuit is permanently connectedacross the inductive device and thus there is no danger ofmalfunctioning or opening of the circuit without the presence of thedischarge circuit. This arrangement also isolates the discharge voltagefrom the power supply system. Further, a two wire control system can beapplied from a make-break controller of other switching means.

The drawing furnished herewith clearly illustrates an embodiment of theinvention incorporating all of the aforementioned features andadvantages as well as others which will be clear from the followingdescription.

In the drawing:

FIG. 1 is a schematic circuit diagram of a lift magnet systemincorporating the subject matter of the present invention;

FIG. 2 shows a portion of the schematic circuit diagram shown in FIG. 1with the circuit energized to eifect a lifting operation and with thecurrent paths schematically shown by arrows;

FIG. 3 is a view similar to FIG. 2 illustrating the paths in thedischarge circuit immediately following the opening of the power circuitto the magnet; and

FIG. 4 is a view similar to FIGS. 2 and 3 showing the circuit at themoment the discharge or drop contacts are closed.

Referring to the drawings and particular-1y to FIG. 1, a lift magnet 1is diagrammatically shown forming a part of a drum-cable unit 2 which isadapted to lower and raise the lift magnet 1 for movement of anysuitable load, not shown. The lift magnet 1 includes an electromagneticcoil 3 connected through a power circuit 4 to a set of incoming powerlines 5 and 6.

In the illustrated embodiment of the invention, the schematic circuit isshown as an across-the-line type diagram with the power lines 5 and 6shown in a vertical position and with the horizontal branch linesnumbered Ll through L-4, respectively, for purposes of simplicity andclarity of explanation. Power lines 5 and 6 are connected to anysuitable direct current source; for example, a combination transformerand full wave rectifier unit.

A control circuit 7 is connected in lines L3 and L-4 and controls a liftcontactor 8 and a drop contactor 9 for controlling the supplying ofpower to circuit 4. The lift contactor 8 is connected in line L3 inseries with a control switch 10 and controls a plurality of associatedcontacts 8-1 through 8-4. Contacts 8-1 are connected in branch line L-limmediately between the power circuit 4 and the power line 5. Contacts8-2 similarly connect branch line L-2 to the opposite power line 6 andthus provide a circuit path from line 5 via line L-1 through circuit 4,line L-2 to line 6.

The drop contactor 9 is connected in line L-4 in series with a set ofnormally closed contacts 8-4 of lift contactor and a set of voltagesensitive relay contacts 11-1 of a relay 11 in circuit 4 for timedactuation, as hereinafter described. The contactor 9 actuates associatedcontacts 9-1 and 9-2 which are connected respectively in lines 1-2 andlines L-l to provide a reverse connection of the power circuit 4 fromline 5, line L-2, circuit 4, line L-l to line 6. Contactor 9 alsoactuates the normally closed contacts 9-3 in line L-3 to preventsimultaneous energization of the contactors 8 and 9.

In the illustrated operation, a voltage sensitive bridge circuit 12 isprovided with the input connected between lines L-1 and L-2, generallyin parallel with the coil 3 and with voltage sensitive relay 11connected across the output.

Before proceeding with a more detailed description of circuit 4, theoperation of the device is briefly described. The switch 10 is closed tocomplete the circuit to the contactor 8 which closes its relatedcontacts 8-1 and 8-2 to supply current to the coil 3. Only a minimalcurrent flows through the bridge circuit 12 for reasons fully developedhereinafter and the magnet coil 3 is provided with maximum energizationfor lifting of a suitable load. When the load is to be dropped, theswitch 10 is opened to deenergize the contactor 8 and open contacts 8-1and 8-2. At that moment, the inductive characteristic of the coil 3establishes a relatively high voltage opposing the decrease in current.This high voltage is applied to the voltage sensitive bridge circuit 12but is isolated from the supply lines 5 and 6 as the contacts of bothcontactors 8 and 9 are opened. The inductive voltage discharges throughthe circuit 12 until a selected voltage level is obtained. At that time,the voltage sensitive relay 11 is energized to close its relatedcontacts 11-1 in line L4 and completes the operating circuit to the dropcontactor 9 which closes the related contacts 91 and 92. As a result, areverse voltage is impressed across the coil 3, causing the currenttherethrough to rapidly drop to zero and thereafter to momentarilymaintain a demagnetizing current. After a selected time period, thedifferential voltage across the bridge 12 drops below the holdingvoltage of the voltage sensitive relay 11 and the contacts 11-1 open. Asa result thereof, contactor 9 is de-energized and contacts 9-1 and 9-2open, returning the circuit to the standby position shown in FIG. 1.

More particularly, referring to FIG. 1, the discharge or bridge circuit12 for controlling the voltage sensitive relay 11 is connected in theform of a wheatstone bridge and includes a pair of fixed resistors 13and 14 forming two legs connected in series between lines L1 and L-2.The central junction 15 of resistors 13 and 14 constitutes an outputterminal which is connected to one side of the voltage sensitive relay11 in series by a pair of normally closed contacts 8-3 of the liftcontactor 8. The contacts 8-3 open during the lifting operation topositively assure that the voltage relay 11 will not be energized duringthe lifting function. The contacts 8-3 may be eliminated if thecharacteristic of the other components hereinafter developed areproperly selected.

The other two legs of the bridge are formed by a nonlinear resistor 16connected in series with a diode 17 and a fixed resistor 18 between thelines L-1 and L-2. The junction 19 between the nonlinear resistor 16 andthe diode 17 constitutes the other output terminal connected to theopposite side of the voltage sensitive relay 11. The diode 17 is biasedwith respect to the connection between lines L-l and L-2 to prevent theflow of current from line L-l through the nonlinear resistor 16. Aresistor 20 is connected in line L-2 between the legs of the bridge 12and is operably inserted in the opposite legs as hereinafter described.

A second nonlinear resistor 21 is connected in parallel with thenonlinear resistor 16 and the diode 17 as well as the series connectionof the resistor 18 and the coil 3 to provide a discharge path uponde-energization of the coil 3.

The illustrated control circuit 7 for controlling the energization andde-energization of the magnet 3 includes the contactor 8 connected inseries circuit with the control switch 10 and a polarizing diode 22. Thediode 22 is desirable to protect the nonlinear resistors. Thus, if thepolarity of the lines and 6 is reversed for any reason, current flowthrough the control circuit is prevented.

The contactor 9 in line L-4 is automatically controlled through the setof normally closed interlocking lift contacts 84 of contactor 8 and thenormally open voltage sensitive relay contacts 11-1 of the voltagesensitive relay 11. Contacts 8-4 positively prevent energization ofcontactor 9 unless contactor 8 has been de-energized to open the relatedlift contacts 81 and 8-2.

The voltage sensitive relay contacts 11-1 provide an automaticenergizing of the contactor 9 to close its related contacts 91 and 9-2for a timed period during which the magnet coil 3 is de-energized andsubsequently demagnetized as a result of the following circuit action.

Referring particularly to FIG. 2, the condition of the circuit isillustrated as a result of the closing of switch in branch line L-3 andthe resulting energization of the contactor 8. Contacts 8-1 and 8-2 areclosed and set up a pair of current circuits with the line 23 indicatingthe main current path through the coil 3. A small current shown by thedotted line 24 may also pass through the resistors 13, 14 and inparallel with the electromagnetic coil 3. Rectifier diode 17 blockscurrent through the associated paths. As a result, maximum power issupplied to the coil 3 to provide the desired lift energization thereof.

To drop the load, the switch 10 is opened to break the circuit to thecontactor 8 and open contacts 8-1 and 82. This will then establish thecircuit shown in FIG. 3. The coil 3 and the related power circuit areisolated from the lines 5 and 6 by the open lift contacts 8-1 and 8-2and the open drop contacts 9-1 and 9-2. Immediately upon opening of thecontacts 81 and S-2, a current which may be of the order of 100 ampereswill be maintained for a short period as a result of the self-inductanceof the coil 3 but will decrease at an exponential rate. The voltagewhich is generated is a direct function of the resistance of thenonlinear resistance 16 and may for example be limited to 650 or 750volts. The current from the magnet coil 3 primarily flows through thenonlinear resistor 16 and diode 17 as shown by full line 25 and alsothrough the circuit of fixed resistor 13, 14 and 20 as shown by thedotted line 26. This will establish an unbalance across the four legs ofthe bridge circuit 12 with the resulting voltage differential betweenthe junctions 15 and 19. The four legs consist respectively of resistor13, resistor 14 in series with resistor 20, resistor 16, and resistor 18in series with diode 17. However, the voltage generated betweenjunctions 15 and 19 is below the voltage at which the relay 11 will pickup. For example, the voltage sensitive relay 11 may be selected to pickup at approximately 30 volts whereas during the initial discharging ofthe coil 3 only 10 volts are produced across the voltage sensitive relay11. The circuit will therefore continue to discharge the energy in thecoil 3 as shown in FIG. 3. This provides a rapid dissipation of theinductive energy and in approximately one second, the current will havedropped to a relatively low value. The resistance of the nonlinearresistor 16 increases as the current decreases and the voltagedistribution shifts and increases the voltage differential betweenjunctions 15 and 19. The voltage across the relay 11 will thereforeincrease to the pickup level. and actuate the voltage sensitive relay 11to close contacts 11-1 in line L-4 and energize contactor 9. Thisresults in closing of contacts 91 in line L-2 and 9-2 in line L-1 toproduce the circuit condition of FIG. 4.

In FIG. 4, a main path 27 is cornpleted from power line 5 throughcontacts 9-1, resistor 20, coil 3, contacts 9-2 to line 6. This forcesthe current to rapidly drop to zero and reverse to pass a demagnetizingcurrent. Durmg the initial portion of this sequence, the bridge circuit12 is energized as follows. A voltage is applied across fixed resistor13 in series with fixed resistor 14, as shown by the dotted line 28. Avoltage also appears across resistor 20 in series with the magnet coil3. Thus, the condition of the bridge circuit 12 is such that theresistor 20 is no longer in the leg including resistor 14 but is now inthe leg including the magnet winding or coil 3. At this point, thenonlinear resistor 16 may not be carrying current upon thecharacteristic of the particular resistor shown. In any event, asufiicient voltage differential is maintained across the voltagesensitive relay 11 to hold it in the energized position. As a result,contactor 9' is maintained energized for a selected period to maintainthe reverse voltage application to the coil 3, produce the rapid drop tozero and actual momentary reversal of current.

A very short period after, perhaps one-half second, the current inmagnet coil 3 has increased to the level where the voltage betweenjunctions 15 and 19a becomes less than the holding voltage of relay 11,which drops out and returns the circuit to standby. The nonlinearresistor 21 functions at this time to discharge the energy in coil 3resulting from the reverse current and limits the voltage.

In practice the nonlinear resistor 16 which functions to limit thevoltage at initial de-energization of coil 3 is a relatively thinelement having a large diameter in order to d1scharge the substantialamount of energy in coil 3.

Rectifier 17 'isolates and thereby protects the resistor 16 anacceptable level.

" The circuit therefore functions to rapidly discharge the energy storedin the magnet coil without creation of dangerous voltage peaks orapplication thereof to the power supply system. The discharge circuit ispermanently connected to provide instantaneous response and produces apositive voltage limitation. The two wire control system of the presentinvention permits complete control by a simple push button withautomatic discharge action upon release thereof. Relays and contactorspresently used in magnetic lift controls can be used in the presentinvention and thus the cost of the system is minimized.

Various modes of carrying out the invention are contemplated as beingwithin the scope of the following claims particularly pointing out anddistinctly claiming the subject matter which is regarded as theinvention.

I claim:

1. A circuit means for rapid discharge and reset of a magnetic loadenergized from a direct current source, comprising power means forapplying oppositely polarized currents to the load and having loadterminals adapted to be connected to the load,

' a discharge circuit connected in parallel with said load terminals andincluding a plurality of current paths at least one of which includesnonlinear impedance means and constructed to impress self-induced loadvoltages upon the nonlinear impedance, said discharge circuit includinga pair of output terminals in different current paths, and

a control means for said power means connected to the output terminalsand responsive to a selected voltage differential therebetween formomentarily applying a reverse current to the inductive load and thedischarge circuit, said nonlinear impedance means serving to initiallydischarge a portion of the energy in the load at a selected maximumvolt-age peak and then changing the voltage distribution in theassociated current paths to establish the selected voltage differential.

2. The circuit means of claim 1 having means in the discharge circuit toessentially isolate incoming power from the nonlinear impedance means.

3. The circuit means of claim 1 including control means for connectingof power to the terminals having polarized means whereby power of aselected polarity if operative and of an opposite polarity isinoperative.

4. The circuit means of claim 3 wherein said polarized means is arectifying means.

5. A circuit means for rapid discharge and reset of an inductive loadenergized from a direct current source, comprising a pair of loadterminals,

a discharge circuit connected in parallel with said load terminals andincluding a plurality of current paths arranged in a bridge circuit withoutput control terminals one each in a different current path, at leastone of said paths having a nonlinear impedance element connected thereinand arranged to impress the self-induced load voltage upon the nonlinearimpedance,

power means for applying oppositely directed currents through the loadand the discharge circuit, and

a voltage responsive switch means for operatively conmeeting said powermeans to the load terminals and having operating means connected to theoutput terminals and responsive to a selected voltage differential tomomentarily apply a reverse current to the load and the dischargecircuit, said nonlinear impedance serving to intially discharge aportion of the energy in the load at a selected maximum voltage peak andthen changing the voltage distribution in the associated current pathsto establish the selected voltage differential.

6. A discharge and reset circuit means for a highly inductive coil suchas a lifting magnet,

a direct current power circuit including said coil and a dischargebridge circuit connected in parallel,

a pair of power lines,

first contactor means having contacts connecting the power circuit tothe pair of power lines to supply lifting power to the coil,

a second contactor means having contacts connecting the power circuit tothe pair of power lines to supply dropping power to the coil,

a diode,

an on-otf switch connecting said first contactor to the power lines inseries with said diode,

said bridge circuit including a plurality of operative legs connectedbetween input terminal means and output terminal means and includingcircuit impedances at least one of which is a nonlinear impedance toprovide automatic current responsive voltage distribution within theoperative legs and to opposite sides of an output terminal,

electroresponsive means connected to the output terminals and havingswitch means connecting said second contactor to the power lines, and

means connecting said coil to the input terminals to discharge theenergy thereof into the bridge circuit upon opening of the firstcontactor with an intitial discharge period during which the nonlinearimpedance adjusts its impedance value to hold the voltage across themagnet to minimum value consistent with rapid operation and increasinginimpedance as the discharging current decreases to create an operativevoltage across the output terminal to automatically energize theelectroresponsive means and thereby actuate the second contactor toreverse the voltage applied to the coil and rapidly reduce the currentthrough the coil to zero and create an oppositely directed demagnetizingcurrent which after a time period creates a voltage differential belowthe operative voltage of the electroresponsive means.

7. A discharge and reset circuit means for a highly inductive coil suchas a lifting magnet,

a power circuit including said coil and a discharge bridge circuit,

control means selectively connecting the power circuit to supply liftingcurrent to the coil and to supply reset current to the coil,

said bridge circuit including a closed circuit loop having fouroperative legs each of which includes circut resistors with outputterminal means at one set of opposite junctions connected to operatesaid control means to supply reset current and having input terminalmeans at the other set of opposed junctions connected in parallel withsaid coil, one of said legs including a nonlinear impedance to provideautomatic current responsive voltage distribution within the fouroperative legs and the adjacent leg forming an output terminal includinga rectifying means to block lifting current through said nonlinearimpedance, and

said coil being connected to discharge into the bridge circuit uponremoval of the lifting current with an initial period during which thenonlinear impedance adjusts its impedance value to hold the voltageacross the coil to maximum peak value consistent with rapid operationand increasing in impedance as the dis- 8.- linear impedance connectedin parallel with the 7 charging current decreases to create an operativevoltage impressed upon said control means to automatically establish thereset current applied to the coil and thereby rapidly reduce the currentto zero and create a demagnetizing current which creates a. differentialacross the output terminals of the bridge circuit actuating after aselected period which releases said control means.

The circuit means of claim 7 including a second nonfirst named nonlinearimpedance and said rectifying means.

A discharge and reset circuit means for a highly inductive coil such asa lifting magnet,

switch means having a first position said bridge circuit including afour legged power circuit including said coil and a discharge bridgecircuit,

connecting the power circuit to supply lifting power to the coil and asecond position connecting the power circuit to supply dropping power tothe coil,

control relay for actuating said switch means to the second position,

closed loop having four operative legs including circuit impedances withoutput terminals at one set of opposed junctions connected to said relaywith input terminals at the other set of opposed junctions connected inparallel with said coil, one of said legs including a nonlinearimpedance to provide automatic current responsive voltage distributionwithin the four operative legs, said bridge circuit including rectifyingmeans to block current through said nonlinear impedance with the switchmeans in the first position, and

means to actuate the switch means to and from said first position forselectively energizing the coil, said coil discharging into the bridgecircuit upon movement of the switch means from the first position withan initial period during which the nonlinear impedance adjusts itsimpedance value to hold the voltage across the magnet to minimum valueconsistent with rapid operation and increasing in impedance as thedischarging current decreases to create an operative voltage impressedupon said relay to automatically establish the second position of theswitch means and reverse the voltage applied to the coil to rapidlyreduce the current to zero and create a demagnetizing current whichcreates a voltage differential below the holding voltage of the relay.

10. A lift and discharge control circuit for an electromagnetic lifthaving a magnet coil wound on a magnetic core,

comprising a pair of direct current power lines,

pair of branch lines connected between said power lines and each havinga lift switch means and a drop switch means to the opposite endsthereof, the switch means in one branch line being reversed with respectto the opposite branch line,

means for connecting the coil between said branch lines for selectiveenergization with a lift current in response to actuation of the liftswitch means and with a drop current in response to actuation of thedrop switch means,

impedance devices serially connected between said impedance unit beingconnected in' series with a second output terminal of the bridge circuitat the junction of the nonlinear resistor and the diode, secondnonlinear resistor connected in parallel with said first nonlinearresistor and said diode,

a fixed impedance in the branch line between the legs of the bridgeincluding one of said first named pairs of fixed resistors and thediode, and

an electroresponsive means connected between said output terminals andhaving contacts connected to control said drop switch means, saidnonlinear resistors being responsive to the discharging currents of thecoil to establish an initial energy discharge period during which thevoltage differential is insufficient to actuate electroresponsive meansand a second discharge period during which the voltage differential issufiicient to operate said electroresponsive means, the drop currentthrough said coil and said bridge circuit maintaining anelectroresponsive means operative until a selected voltage is createdacross said coil.

11. The control circuit of claim 10 having a circuit switching meanscomprising an electroresponsive means switch means and being connectedcontacts.

12. The control circuit of claim 10 having a safety switch meansconnected in circuit with said electroresponsive means to positivelyprevent energization thereof and actuated with the lift switch means.

13. A lift and discharge control circuit for an electromagnetic lifthaving a magnet coil wound on a magnetic core, comprising a pair ofdirect current power lines,

a pair of branch lines connected between said power lines and eachhaving a lift switch means and a drop switch means to the opposite endsthereof, the location of the switch means being reversed in therespective branch lines,

means for connecting the coil between said branch lines for selectiveenergization with a lift current in response to actuation of the liftswitch means and with a drop current in response to actuation of thedrop switch means,

a pair of fixed resistors serially connected between said branch linesto form two legs of a bridge circuit and having a first output terminalat the junction of the resistors,

a nonlinear resistor forming a third bridge leg connected in series witha diode and a fixed resistor forming a fourth bridge leg connectedbetween said branch lines with a second output terminal of the bridgecircuit ,at the junction of the two legs,

a second nonlinear resist-or connected in parallel with said firstnonlinear resistor and said diode,

a fixed resistor in the branch line between the legs of the bridgeincluding one of said first named pairs of fixed resistors and thediode, and

a relay connected between said output terminals and having contactsconnected to control said drop switch means, said relay being selectedwith a pull-in voltage in excess of the voltage differential establishedwith the lift switch means closed and a selected holding voltage, saidnonlinear resistors being responsive to the discharging currents of thecoil to establish an initial energy discharge period during which thevoltage differential is below said pull-in voltage and a seconddischarge period during which the voltage differential is above saidpull-in voltage, the drop current through said coil and said bridgecircuit maintaining a holding relay voltage differential until aselected voltage is created across said coil.

for actuating said drop in circuit with said References Cited UNITEDSTATES PATENTS 2,181,539 11/1939 Wertz 3l7l23 2,648,033 8/1953 Hudson3l7l23 X MILTON O. HIRSHFIELD, Primary Examinerr 7 L. T. HIX, AssistantExaminer.

1. A CIRCUIAT MEANS FOR RAPID DISCHARGE AND RESET OF A MAGNETIC LOADENERGIZED FROM A DIRECT CURRENT SOURCE, COMPRISING POWER MEANS FORAPPLYING OPPOSITELY POLARIZED CURRENTS TO THE LOAD AND HAVING LOADTERMINALS ADAPTED TO BE CONNENCTED TO THE LOAD, A DISCHARGE CIRCUITCONNECTED IN PARALLEL WITH SAID LOAD TERMINALS AND INCLUDING APLURALITYOF CURRENT PATHS AT LEAST ONE OF WHICH INCLUDES NONLINEAR IMPEDANCEMEANS AND CONSTRUCTED TO IMPRESS SELF-INDUCED LOAD VOLTAGES UPON THENONLINEAR IMPEDANCE, SAID DISCHARGE CIRCUIT INCLUDING A PAIR OF OUTPUTTERMINALS IN DIFFERENT CURRENT PATHS, AND A CONTROL MEANS FOR SAID POWERMEANS CONNECTED TO OUTPUT TERMINALS AND RESPONSIVE TO A SELECTED VOLTAGEDIFFERENTIAL THEREBETWEEN FOR MOMENTARILY APPLYING A REVERSE CURRENT TOTHE INDUCTIVE LOAD AND THE DISCHARGE CIRCUIT, SAID NONLINEAR IMPEDANCEMEANS SERVING TO INITIALLY DISCHARGE A PORTION OF THE ENERGY IN